In the field of optical communication, a wavelength-division multiplex transmission system has been studied wherein a plurality of signals are put respectively on light in its plurality of different wavelengths and the light loaded with the plurality of signals are transmitted through one optical fiber to increase communication capacity. In this system, an optical wavelength-division multiplexer/demultiplexer for multiplexing or demultiplexing a plurality of lights having different wavelengths plays an important role. Among others, an optical wavelength multiplexer/demultiplexer using an array wavelength diffraction grating can increase the number of wavelengths multiplexed at a narrow wavelength spacing, and hence is promising.
For the optical wavelength multiplexer/demultiplexer used in the wavelength-division multiplexed transmission system, broadening and flattening of the wavelength transmission band, steep rise and fall in the transmission band are important in connection with the wavelength control tolerance of a semiconductor laser source, gain characteristics of an optical fiber amplifier, wavelength characteristics of a dispersive compensating fiber and the like. An optical wavelength multiplexer/demultiplexer which brings an electric field distribution of signal light to a near rectangular form at the interface of input waveguides and an input slab waveguide has been proposed in order to broaden the transmission band. In this connection, the applicant relating to the invention has proposed in Japanese Patent Application No. 122577/1996 an optical wavelength multiplexer/demultiplexer comprising a slit in input waveguides, and K. Okamoto and A. Sugita; Flat spectral response arrayed-waveguide grating multiplexer with parabolic waveguide horns; ELECTRONICS LETTERS, vol. 32 No. 18 pp 1661 to 1662 proposes an optical wavelength multiplexer/demultiplexer wherein the width of the input waveguide is parabolically increased toward the input slab waveguide.
According to the optical wavelength multiplexer/demultiplexer which brings an electric field distribution of signal light to a near rectangular form at the interface of input waveguides and an input slab waveguide, the wavelength characteristics determined by superimposition integral of the electric field distribution of the optical signal and the natural mode of the output waveguides are such that the loss is increased although the transmission band is broadened. Further, since the electric field distribution of an optical signal in a connecting section between the input waveguides and the input slab waveguide is not fully in a rectangular form, the broadening of the wavelength transmission band, the flattening, and the steep rise and fall in the transmission band are unsatisfactory. Therefore, a change in wavelength in a light source unfavorably results in a change in loss.